The present invention relates to optical switch matrices and, more particularly, to an improved optical switch matrix with wraparound architecture
FIG. 1 illustrates the prior art optical switch matrix 10 of which the present invention is an improvement. This prior art optical switch matrix also is described as prior art in U.S. Pat. No. 4,852,958, to Okuyama et al. Matrix 10 connects four input waveguides 18 to four output waveguides 20 via four rows (a, b, c, d) of switches. Each row includes a 1.times.2 switch 12, two 2.times.2 switches 14 and a 2.times.1 combiner 16. Each 1.times.2 switch 12 has a single input port 40 and two output ports: an upper output port 22 and a lower output port 24. Each 2.times.2 switch has two input ports and two output ports: an upper input port 26, a lower input port 28, an upper output port 30 and a lower output port 32. Each 2.times.1 combiner has two input ports: an upper input port 34 and a lower input port 36; and a single output port 42. Input waveguides 18 are connected to corresponding input ports 40. Output waveguides 20 are connected to corresponding output ports 42. In each row, lower output ports 24 and 32 are connected by intermediate waveguides 38 to lower input ports 28 and 36 of the immediately succeeding switches 14 or 16; whereas upper output ports 22 and 30 are connected by intermediate waveguides 38 to upper input ports 26 or 34 of respective switches 14 or 16 in the cyclically succeeding row. Cyclical succession means that the connection topology is as though the rows were fabricated on the periphery of a cylinder, parallel to the axis of the cylinder: row b is the successor of row a, row c is the successor of row b, row d is the successor of row c and row a is the successor of row d. So, for example, an intermediate waveguide 38 connects upper output port 22 of switch 12d to upper input port 26 of switch 14aa. In Okuyama et al., rows a and d are shown connected by intermediate waveguides 38 that cross other intermediate waveguides 38. For illustrational clarity, this wraparound of the connectivity between rows a and d is represented in FIG. 1 by the circled terminations A, B and C on intermediate waveguides 38 that connect output ports 22 and 30 in row d to input ports 26 and 34 in row a.
Several implementations of 2.times.2 switches 14 are known in the prior art, including, among others, directional coupler switches and Mach-Zehnder interferometer switches. A 2.times.2 switch 14 can be in one of two states: a straight-through state (also called the "bar" state or the "=" state), in which optical energy, that enters switch 14 via upper input port 26, exits switch 14 via upper output port 30, and in which optical energy, that enters switch 14 via lower input port 28, exits switch 14 via lower output port 32; and a crossover state (also called the "cross" state or the "X" state") in which optical energy, that enters switch 14 via upper input port 26, exits switch 14 via lower output port 32, and in which optical energy, that enters switch 14 via lower input port 28, exits switch 14 via upper output port 30. Switch 14 is switched from one state to another by the application of a voltage to an internal component of switch 14. With no voltage applied, switch 14 is "OFF" in one of its two states. With the switching voltage applied, switch 14 is "ON" in the other of its two states. Two variants of switch 14 thus are possible. In the first variant, switch 14 is in its=state when OFF and in its X state when ON. In the second variant, switch 14 is in its X state when OFF and in its=state when ON. In the context of the present invention, the first variant of switch 14 is preferred.
2.times.2 switch 14 is turned into a 1.times.2 switch 12 simply by making one of the input ports an idle port, ie., leaving this input port disconnected. For example, if lower input port 28 is idle, then upper input port 26 serves as input port 40, upper output port 30 serves as upper output port 22 and lower output port 32 serves as lower output port 24. In the preferred variant of such a 1.times.2 switch 12, when this switch 12 is OFF, it is in its=state, so that optical energy entering via input port 40 leaves via upper output port 22; and when this switch 12 is ON, it is in its X state, so that optical energy entering via input port 40 leaves via lower output port 24. Alternatively, if input port 26 is idle, then lower input port 28 serves as input port 40. In the preferred variant of this alternative 1.times.2 switch 12, when this switch 12 is OFF, it is in its=state, so that optical energy entering via input port 40 leaves via lower output port 24, and when this switch 12 is ON, it is in its X state, so that optical energy entering via input port 40 leaves via upper output port 22.
2.times.1 combiners 16 may be either passive or active. 2.times.2 switch 14 is turned into a 2.times.1 active combiner 16 simply by malting one of the output ports an idle port, i.e., leaving this output port disconnected. For example, if lower output port 32 is idle, then upper input port 26 serves as upper input port 34, lower input port 28 serves as lower input port 36, and upper output port 30 serves as output port 42. In what follows, an active 2.times.1 combiner usually is referred to as a "2.times.1 switch". In the preferred variant of such a 2.times.1 switch 16, when this 2.times.1 switch 16 is OFF, it is in its=state, so that only optical energy entering via upper input port 34 leaves via output port 42; and when this 2.times.1 switch 16 is ON, it is in its X state, so that only optical energy entering via lower input port 36 leaves via output port 42. Alternatively, if upper output port 30 is idle, then lower output port 32 serves as output port 42. In the preferred variant of this alternative 2.times.1 switch 16, when this 2.times.1 switch 16 is OFF, it is in its=state, so that only optical energy entering via lower input port 36 leaves via output port 42, and when this 2.times.1 switch 16 is ON, it is in its X state, so that only optical energy entering via upper input port 34 leaves via output port 42. Although 2.times.1 combiners 16 are most simply implemented as passive combiners, such as y-junction combiners, the preferred 2.times.1 combiners of the present invention are active 2.times.1 combiners, both because passive 2.times.1 combiners are inherently lossy and for a second reason describe below.
By turning appropriate switches 12 and 14 ON and OFF, any input waveguide 18 may be connected to any output waveguide 20. For example, let 1.times.2 switches 12 be 2.times.2 switches with idle lower input ports, let 1.times.2 switches 12 and 2.times.2 switches 14 be in their=states when OFF and in their X states when ON, and let 2.times.1 combiners 16 be passive. With all switches 12 and 14 OFF, input waveguide 18a is connected to output waveguide 20d, input waveguide 18b is connected to output waveguide 20a, input waveguide 18c is connected to output waveguide 20b, and input waveguide 18d is connected to output waveguide 20c. Turning switch 12a ON connects input waveguide 18a to output waveguide 20a. Turning switch 14ba ON connects input waveguide 18a to output waveguide 20b. Turning switch 14cb ON connects input waveguide 18a to output waveguide 20c.
By using active 2.times.1 combiners 16, optical switch matrix 10 may be configured so that no input waveguide 18 is connected to any output waveguide 20 unless a switch 12, 14 or 16 is turned ON. For example, let 1.times.2 switches 12 and 2.times.2 switches 14 be as above, and let 2.times.1 combiners 16 be 2.times.2 switches, with idle upper output ports, that are in their=states when OFF and in their X states when ON. Now, with all switches 12 and 14 OFF, switch 16d must be turned ON to connect input waveguide 18a to output waveguide 20d, switch 16a must be turned ON to connect input waveguide 18b to output waveguide 20a, switch 16b must be turned ON to connect input waveguide 18c to output waveguide 20b, and switch 16c must be turned ON to connect input waveguide 18d to output waveguide 20c.